1. Field of the Invention
The field of the present invention relates to a photovoltaic solar module, a process for the preparation thereof, and a device for generating electric energy employing such a solar module.
2. Background
Solar modules are construction elements for the direct generation of electricity from sunlight. Key factors for a cost-efficient generation of solar electricity include the efficiency of the solar cells employed as well as the production cost and durability of the solar modules.
A solar module usually consists of a framed composite of glass, interconnected solar cells, an encapsulation material and a backside construction. The individual layers of the solar module serve the following functions.
The front glass serves for protection from mechanical impact and the effects of the weather. It must have an excellent transparency in order to keep absorption losses in the optical spectral range of from 300 nm to 1150 nm and thus efficiency losses of the silicon solar cells, which are usually employed for power generation, as low as possible. Normally, tempered low-iron white glass (3 or 4 mm thick), whose transmittance in the above spectral range is around 90 to 92%, is used.
The encapsulating material (mostly EVA (ethylene-vinyl acetate) sheets are used) serves for adhesively bonding the whole module assembly. During a lamination process, EVA melts at about 150° C., flows into the spaces of the soldered solar cells and is thermally cross-linked. The formation of air bubbles, which would result in reflection losses, is avoided by lamination under vacuum.
The backside of the module protects the solar cells and the encapsulating material from moisture and oxygen. In addition, it serves as a mechanical protection from scratch etc. when the solar modules are mounted, and as an electrical isolation. Another sheet of glass or a composite sheet can be employed as the backside construction. Mostly, the variants PVF(polyvinyl fluoride)-PET(polyethylene tere-phthalate)-PVF or PVF-aluminum-PVF are employed.
In particular, the encapsulating materials employed in solar module construction must have good barrier properties against humidity and oxygen. Humidity and oxygen do not attack the solar cells themselves, but corrosion of the metal contacts and chemical degradation of the EVA encapsulating material occur. A destroyed solar cell contact leads to complete failure of the module since normally all solar cells in one module are electrically serially connected. A degradation of the EVA can be seen from a yellowing of the module associated with a corresponding performance reduction by light absorption and visual deterioration. Today, about 80% of all modules are encapsulated on the backside with one of the composite sheets described, and glass is used for the front and back sides of about 15% of the solar modules. In this case, in part highly transparent casting resins, which cure slowly, however (several hours), are employed as encapsulating material instead of EVA.
In order to achieve competitive electricity generation costs of solar electricity despite the relatively high investment cost, solar modules must reach long service lives. Therefore, solar modules are designed for a service life of 20 to 30 years today. In addition to a high weather stability, high demands are placed on the temperature resistance of the modules, whose temperature can vary cyclically during operation from 80° C. under full solar irradiation to temperatures below the freezing point. Accordingly, solar modules are subjected to extensive stability tests (standard tests according to IEC 61215 and IEC 61730), which include weather tests (UV irradiation, damp heat, temperature cycling), but also hail impact test and tests of the electric insulation performance.
Module finishing accounts for 30% of the total cost for photovoltaic modules, which is a relatively large proportion. This large proportion of module fabrication is due to high material costs (including for the encapsulating material, frame, backside multilayer sheet) and long process times, i.e., low productivity. The above described individual layers of the module composite are frequently still manually assembled and oriented. In addition, the relatively slow melting of the EVA hot-melt adhesive and the lamination of the module composite at about 150° C. under vacuum cause cycle times of about 20 to 30 minutes per module.
Due to the relatively thick front glass sheet, conventional solar modules additionally have a high weight, which in turn necessitates stable support constructions, which are expensive. Also, the problem of heat dissipation is unsatisfactorily solved in current solar modules. Upon full solar irradiation, the modules will heat up to 80° C., which results in a temperature-induced deterioration of the solar cell efficiency and thus ultimately in solar electricity becoming more expensive.
In the prior art, solar modules are mainly used with a frame of aluminum. Although aluminum is a light metal, its weight contributes substantially to the total weight. Just with larger modules, this is a drawback that requires expensive support and attachment constructions.
In order to prevent the ingress of water and oxygen, said aluminum frames have an additional seal on their interior side facing towards the solar module. In addition, there is another disadvantage in that aluminum frames are prepared from rectangular profiles, so that their shapes are severely limited.
To reduce the solar module weight, to avoid an additional sealing material and to increase the freedom of design, U.S. Pat. No. 4,830,038 and U.S. Pat. No. 5,008,062 describe the provision of a plastic frame around the corresponding solar module, the frame being obtained by the RIM (reaction injection molding) process.
Preferably, the polymeric material employed is an elastomeric polyurethane. Said polyurethane preferably has a modulus of elasticity within a range of from 200 to 10,000 psi (corresponding to about 1.4 to 69.0 N/mm2).
Various possibilities for reinforcing the frame are described in these two patent specifications. Thus, reinforcing components made of, for example, a polymeric material, steel or aluminum can be integrated with the frame when the latter is formed. Also, fillers can be included in the frame material. These may be, for example, plate-like fillers, such as the mineral wollastonite, or acicular/fibrous fillers, such as glass fibers.
Similarly, DE 37 37 183 A1 also describes a process for the preparation of the plastic frame of a solar module, the Shore hardness of the material employed preferably being adjusted to ensure a sufficient rigidity of the frame and an elastic accommodation of the solar generator.
DE 10 2005 032 716 A1 describes flexible solar modules in which the frame has a permanently elastic flexible consistency. It is necessary to adjust the rigidity of the plastic material low and to substantially dispense with fillers, so that the frame itself remains flexible.
Due to the different coefficients of thermal expansion of polyurethane and glass and due to the significant shrinkage of the polyurethane systems, delaminations and ingress of moisture into the interior region of the solar module occurred again and again in the past, which ultimately resulted in the module being destroyed.
Solar modules inserted in roof constructions must meet the requirements of DIN 4102-7 in accordance with the German Building Code. In particular, they must prove their resistance against flying sparks and radiant heat.
In view of these drawbacks of the prior art, a solar module should have a sufficient long term stability which prevents delaminations and/or the ingress of moisture from occurring. Such a solar module should be able to be handled without problems. For this purpose, it should have a sufficient rigidity, but not too low of an elongation at break in order that it is not destroyed immediately upon a low impact stress (for example, from edge chipping when being mounted on a building site). Further, such a solar module should have sufficient flame retardancy.